A variety of techniques and apparatus have been used to satisfy the requirements of vehicle communication systems. For example, in a vehicle communications system including a stationary antenna communicating with a plurality of mobile antennas, one such technique is to employ a radiating cable antenna as the stationary part of the system and dipole antennas as the mobile part of the system. Generally, radiating cable antennas consist of "leaky" coaxial cables having inner and outer conductors separated by a dielectric material, in which the outer conductor is provided with either a continuous slot or a row of apertures extending lengthwise along the cable. In cables including a row of apertures, many apertures are typically provided per wavelength in order to physically approximate a continuous slot. In either case, the slot or apertures serve to couple electromagnetic signals radiating within the cable to fields radiating outside of the cable, such that the cable may be used as a distributed antenna for transmitting or receiving electromagnetic energy.
The communications coverage area supported by a radiating cable is dependent on the length of the cable, the attenuation of the radiated signal along the length of the cable and the transfer efficiency (or conversely, "coupling loss") between the radiating cable and the receiving antenna. Generally, in a vehicle communication system, the length of the radiating cable is relatively long in order to support a correspondingly large coverage area. Attenuation of the signal will increase in proportion to the length of the cable. Typically, to ensure adequate signal strength along the entire length of the cable, the signal is amplified by a series amplifier positioned along the length of the cable. Because these amplifiers are very expensive, reducing the number of required amplifiers would significantly benefit the design and cost of a radiating-cable to vehicle communication system. This may be accomplished by increasing the transfer efficiency (i.e. reducing the coupling loss) between the radiating cable and the receiving antenna without unduly increasing the cable attenuation.
Another design consideration which would significantly benefit radiatingcable to vehicle communication systems is to improve the consistency of the received signal level. Where dipole antennas are used as the mobile part of the system, it has been determined that the signal received by the dipole can vary approximately 7 to 9 dB with small vehicle movements. Such large variations in the received signal level in response to small vehicle movements necessitates the use of a receiver having a large dynamic range and fast time response and contributes to the degradation of the information being transmitted.
In view of the above, there is a need for a vehicle communications system which increases the transfer efficiency between a stationary radiating cable antenna and a mobile receiving antenna without unduly increasing the cable attenuation, thereby reducing the required number of amplifiers in the system. Moreover, the system should support a received signal level which does not significantly vary in response to small vehicle movements. The present invention is directed to addressing each of the aforementioned needs.